Photolithography is a process widely used in semiconductor industry to fabricate electronic devices which uses light to transfer a geometric pattern from a photomask to a substrate such as a silicon wafer. In a photolithography process, a photoresist layer is first formed on the substrate. The photoresist is exposed through a photomask with a desired pattern to a source of actinic radiation. The exposure causes a chemical reaction in the exposed areas of the photoresist and creates a latent image corresponding to the mask pattern in the photoresist layer. The photoresist is next developed in a developer, usually an aqueous base solution, to form a pattern in the photoresist layer. The patterned photoresist can then be used as a mask for subsequent fabrication processes on the substrate, such as deposition, etching, or ion implantation processes.
Two types of photoresist have been used in photolithography: positive resist and negative resist. A positive resist is initially insoluble in the developer. After exposure, the exposed region of the resist becomes soluble in the developer and is then selectively removed by the developer during the subsequent development step. A negative resist, on the other hand, is initially soluble in the developer. Exposure to radiation causes the exposed region of the resist to become insoluble in the developer. During the subsequent development step, the unexposed region of the negative resist is selectively removed by the developer, leaving the exposed region on the substrate to form a pattern.
In a photolithography process, exposure of the photoresist layer to the activation radiation is an important step in attaining a high resolution photoresist image. However, reflection of the activation radiation from the photoresist and the underlying substrate substantially limits the resolution of the process. Two major problems of the reflected radiation are: (1) thin film interference effects or standing waves, which are caused by variations in the total light intensity in the photoresist film as the photoresist thickness changes; and (2) reflective notching, which occurs when the photoresist is patterned over substrates containing topographical features.
The reflected radiation from the photoresist and the underlying substrate has become increasingly detrimental to the lithographic performance of the photoresist under high numerical aperture (NA) and short wavelength (such as 248 nm, 193 nm, and shorter wavelengths) exposure conditions. In implanting levels, the detrimental effect of the reflected radiation is even more pronounced due to the existence of surface topography generated after gate patterning and/or use of various reflective substrates, such as silicon, silicon nitride and silicon oxide, for advanced semiconductor devices.
Both top antireflective coatings (TARCs) and bottom antireflective coatings (BARCs) have been used in the industry to control the reflected radiation and to improve the lithographic image of the photoresist. The reflectivity control provided by a TARC layer is in general not as good as that obtained with a BARC layer. Using a BARC layer, however, generally requires an etch step to remove the BARC layer in order to transfer the resist pattern into the substrate. The etch step could cause resist thinning, wreck damages to the substrate, and affect the performance of the final device. The additional etch step to remove the BARC layer also increases cost and operational complexity in photolithography.
Recently, DBARC materials have been used to alleviate the reflectivity control issues with some success. DBARC materials are removable by the developer in the development step, eliminating the need of the additional etch step. However, most known DBARC materials in the art are compatible only with positive photoresists. The DBARC materials for positive photoresists become soluble to the developer upon radiation in the same manner as the positive photoresists.
Many implanting levels in semiconductor manufacturing employ negative photoresists because negative photoresists provide superior lithographic performance over topography and have less resist shrinkage during ion implantation compared with positive photoresists. Thus, there remains a need for a BARC composition that is developable in a developer, compatible with the overlying negative photoresist, and has desired optical properties so that it can be used as a BARC suitable especially for implanting levels.